This invention relates to a particularly inexpensive and reliably manufacturable circuit for a digital receiver implementing frequency-shift-keyed (FSK) data transmission.
Frequency-shift-keyed (FSK) data transmission, in which digital bits are transmitted using a burst of a first frequency for a xe2x80x9c1xe2x80x9d and a similar burst of a second frequency for a xe2x80x9c0xe2x80x9d, is well known. Typically the xe2x80x9chighxe2x80x9d tone is used to transmit a xe2x80x9c1xe2x80x9d (referred to a xe2x80x9cmarkxe2x80x9d in teletype parlance) and the xe2x80x9clowxe2x80x9d tone to transmit the xe2x80x9c0xe2x80x9d (or xe2x80x9cspacexe2x80x9d), but the invention is not thus limited. The present invention was implemented in connection with using FSK techniques to perform data transmission between cable television xe2x80x9chead endxe2x80x9d installations and telemetry transponders, but is not thus limited. It will be appreciated, however, by those of skill in the art that any FSK information forming part of a cable TV signal will by necessity have been transmitted together with literally hundreds of different video and audio signals, and will be subject to interference from innumerable sources, so that the accurate detection and demodulation of the FSK signal requires thorough and complete processing.
The basic functions required for an FSK receiver (more particularly, of an optimal receiver for FSK signals having a modulation index greater than 1, as understood by those of skill in the art) are shown in schematic form by FIG. 1. The FSK input signal, consisting of a sequence of tones of two different frequencies (likely mixed with numerous other signals and random noise, as indicated above), is provided at 10; it is split and supplied to two bandpass filters 12 and 14, each optimized to pass either the xe2x80x9chighxe2x80x9d or xe2x80x9clowxe2x80x9d tone. Where the filters are optimized to also select for the anticipated shape of the tone pulse, they are referred to as xe2x80x9cmatchedxe2x80x9d filters. The outputs from the filters are passed to detectors 16 and 18, which provide output signals the amplitude of which is proportional to the amount of energy passing through the corresponding filter. These output signals are effectively compared in a summing node 20, which is typically arranged to provide a positive output when the energy passing through the filter (e.g., 12) corresponding to the high tone exceeds that passing through the filter (14) corresponding to the low tone, and a negative output when the energy levels are reversed.
Typical techniques for implementing the circuit of FIG. 1 involve complex digital signal processing (DSP) techniques, which require costly and complex circuit components. It is obviously desirable to reduce the cost of receivers (and possibly other devices) implementing FSK techniques insofar as possible.
It is therefore an object of the invention to reduce the cost of FSK receivers, while suffering no performance penalty.
The above object of the invention, and others which will appear below as the discussion proceeds, are met by the present invention. According to the invention, the input signal is mixed with a local oscillator signal chosen so that the intermediate frequency which results bears a specified mathematical relationship to the sampling frequency of an analog-to-digital (A/D) converter used to digitize the input signal. This allows various signal processing functions to be implemented using simple digital logic rather than relatively complex digital multiplication steps, and in turn allows implementation of the circuit at low cost and with high reliability. In-phase and quadrature phase versions of the input signal are processed simultaneously. Phase linear filters are used, which render the output signal as reliable as possible. Further improvements are realized by use of a truncated infinite series to closely approximate non-linear square and square root functions, providing further simplification.